Hot rolling: Difference between revisions
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'''Hot rolling''', also called '''hot working''', is a [[Metallurgy|metallurgical]] process of in which large pieces of [[metal]] such as slabs or billets are deformed between roller at high temperature to form thinner cross sections. Hot rolling produces thinner cross sections |
'''Hot rolling''', also called '''hot working''', is a [[Metallurgy|metallurgical]] process of in which large pieces of [[metal]] such as slabs or billets are deformed between roller at high temperature to form thinner cross sections. Hot rolling produces thinner cross sections than cold rolling processes with the same number of stages. Hot rolling will reduce the average [[grain size]] of a metal while maintaining an equiaxed [[microstructure]] where as cold rolling will produce a hardened lamellar microstructure. |
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==Process== |
==Process== |
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Revision as of 15:15, 4 August 2008
Hot rolling, also called hot working, is a metallurgical process of in which large pieces of metal such as slabs or billets are deformed between roller at high temperature to form thinner cross sections. Hot rolling produces thinner cross sections than cold rolling processes with the same number of stages. Hot rolling will reduce the average grain size of a metal while maintaining an equiaxed microstructure where as cold rolling will produce a hardened lamellar microstructure.
Process
A slab or billet is passed or deformed between a set of work rolls and the temperature of the metal is generally above its recrystallization temperature, as opposed to cold rolling, which takes place below this temperature. Hot rolling permits large deformations of the metal to be achieved with a low number of rolling cycles. As the rolling process breaks up the grains, they recrystallize maintaining an equiaxed structure and preventing the metal from hardening. Hot rolled material typically does not require annealing and the high temperature will prevent residual stress from accumulating in the material resulting better dimensional stability than cold worked materials.
Hot rolling is primarily concerned with manipulating material shape and geometry rather than mechanical properties. This is achieved by heating a component or material to its upper critical temperature and then applying controlled load which forms the material to a desired specification or size.
Applications
Hot rolling is used mainly to produce sheet metal or simple cross sections from billets.
Hot rolling is primarily concerned with manipulating material shape and geometry rather than mechanical properties. This is achieved by heating a component or material to its upper critical temperature and then applying controlled load which forms the material to a desired specification or size.
Mechanical properties of the material in its final 'as-rolled' form are a function of:
- material chemistry,
- reheat temperature,
- rate of temperature decrease during deformation,
- rate of deformation,
- heat of deformation,
- total reduction,
- recovery time,
- recrystallisation time, and
- subsequent rate of cooling after deformation.
Types of Rolling Mills
Prior to continuous casting technology, ingots were rolled to approximately 200mm in thickness in a slab or bloom mill. Blooms have a nominal square cross section, whereas slabs are rectangular in cross section.
Slabs are the feed material for Hot Strip Mills or a Plate Mill and blooms are rolled to billets in a Billet Mill or large sections in a Structural Mill.
The output from a Strip Mill is coiled and, subsequently, used as the feed for a Cold Rolling Mill or used directly by fabricators. Billets, for rerolling, are subsequently rolled in either a Merchant, Bar or Rod Mill.
Merchant or Bar Mills produce a variety of shaped products such as angles, channels, beams, rounds (long or coiled) and hexagons. Rounds less than 16mm in diameter are more efficiently rolled from billet in a Rod Mill.